This invention relates in one aspect to a process for foaming foamable thermoplastic resinous sheets or preforms. In a more particular aspect, it relates to a method for making thick, low density foam sheets in a submersion foaming process.
Methods to produce foam products from preformed foamable sheets of various heat plastifiable thermoplastic resins such as, for example, olefin polymer resins including polyethylene, ethylene-vinyl acetate, chlorinated polyethylene, polypropylene, polybutadiene and other crosslinkable thermoplastics are generally known in the art. In such methods, the thermoplastic resin to be employed is typically compounded with a so-called chemical or thermally decomposable blowing agent, such as azodicarbonamide, p-toluene sulfonyl semicarbazide, dinitrosopentamethylenetetraamine, etc. Upon heating, the blowing agent decomposes releasing normally gaseous decomposition products. Other additives are commonly combined with the resin and blowing agents. These include zinc stearate and zinc oxide, which help activate foaming and also calcium stearate and Paraplex G-60, which stabilize the heated resins. The resin, blowing agent and additives are then fabricated into a non-cellular sheet or preform, which can then be crosslinked by any of several known methods. Those methods include using .beta.-radiation or .gamma.-radiation (either of which can be used alone or in conjunction with a crosslinking promotor such as trimethylol propane triacrylate, which, if desired, can be compounded into the foamable compositions at the same time as the above-mentioned chemical blowing agent), as well as chemical crosslinking agents, such as dicumyl peroxide 2,5-dimethyl-2,5-di-(tertiary butyl peroxy) hexane or Lupersol-130. Finally, in the actual foaming step itself, the crosslinked preform is heated to a temperature above the decomposition temperature of the blowing agent, causing expansion to a foamed resin product having gas-filled predominately closed cells.
In certain of the prior art foaming methods, the preform, prior to actual foaming, is preheated to temperatures well below the decomposition temperature of the blowing agent to increase the speed of the foaming step.
It is generally known in the art that for preheating and foaming purposes the preform can be placed in or passed into and through the heating medium employed in either a continuous or a batch-wise mode, depending on the dimensions of the preform to be foamed, the dimensions of the heating medium and the equipment available. In a batch-wise foaming process, the preform is of relatively smaller dimensions and the entire preform can be completely contained in the heating medium, residing therein for the amount of time required to achieve the desired degree of foaming. The entire preform is then completely removed from the heating medium. In a continuous foaming method, the preform typically has a much greater length than its diameter, width or thickness depending on the preform shape. The residence time in said continuous method of operation depends on the combination of the linear speed of the preform through the heating medium and the length of the path taken by the preform through the heating medium. In said continuous method, preform can be supplied to the heating medium either directly after extrusion or after having been stored in a convenient manner subsequent to extrusion and, in either case, a crosslinking step can be inserted anywhere in the process prior to foaming. Combinations of continuous and batchwise modes of operation can also be employed.
Several methods of preheating and heat foaming the foamable resin preforms are known in the art and are combined or used independently in an effort to produce uniformly foamed products. These methods include using hot gases, hot liquids and/or radiant heat sources, such as infrared heaters to heat the preform.
In U.S. Pat. No. 3,711,584 to Sagane et al., there is described a process of preheating and foaming a foamable crosslinked thermoplastic resin preform strip using a combination of hot air and infrared heaters to foam said preform as it continuously falls vertically through a foaming chamber. In such process, the preform is preheated primarily to control the point at which the rapid foam expansion takes place subsequently in the foaming chamber.
In U.S. Pat. No. 3,562,367 to Shinohara et al. there is described a process of heat foaming a foamable crosslinked thermoplastic resin preform using a heated liquid bath to float and heat the preform from below while heating it from above with infrared radiation.
Among other imperfections in these methods, the heated polymer is usually in direct contact with air. This can result in oxidation and/or degradation of some polymers, unless an inert atmosphere is substituted and maintained in the foaming area. However, such addition to the process is expensive and inconvenient.
In U.S. Pat. No. 4,143,106 to Coyne and U.S. Pat. No. 4,155,965 to Allada, there are described processes to avoid the problems encountered in the above processes. The Coyne patent teaches a method for foaming a foamable thermoplastic resin preform by floating such preform upon a heated liquid bath while heating it from above by flooding the upper surface of the floating preform with a blanket of liquid at substantially the same temperature as the heated bath. The Allada patent teaches a process for heat foaming foamable preforms by completely submerging them in a heated liquid bath. In such process, the foam is kept submerged by using an endless belt moving at a speed faster than the foam transport speed to generate a dynamic fluid layer between foam and belt. This layer allows transport of the foam while it is sticky and has little tensile strength. In the Allada patent, the preforms can be preheated by a separate preheater or upon initial submersion in the liquid bath. In the Coyne patent, however, a preheating step is not used.
Even with improved processes of the Coyne and Allada patents, certain unsolved problems remain, such as, for example, the difficulty of obtaining uniform heating throughout the preform during foaming and the attendant difficulty of attaining the desired degree and uniformity of foaming in a commercially reasonable time. Moreover, such problems are particularly pronounced when attempting to produce products of low density, (e.g., less than 8 pounds per cubic foot), and of relatively thick cross-section minimum dimension (e.g., more than three-eighths inch). Foams of those sorts are especially prone towards distorting and even tearing during the foaming process.
In view of the foregoing, it is an object of this invention to provide an improved and economical method of foaming heat foamable thermoplastic resin preforms. A particular object is to provide an improvement whereby a submersion foaming process can be used to foam a thick thermoplastic resin preform and produce a thick, low density foam therefrom.